The major functions of an exterior wall include the aesthetic design provided by the project architect and the interior environmental protection design provided by the exterior wall system designer or supplier. It is well recognized in the industry that wind load resistance and water-tightness performance are the two most important functions in the interior environmental protection design. Since an exterior wall is formed from many wall panels, there are many horizontal and vertical panel joints known as the grid lines in the exterior wall. The grid line design becomes a major feature in the aesthetic design created by the architect. Therefore, the architect demands as much freedom as possible for creating the grid line design. However, the grid line design has major functional impacts, most notably on wind load resistance and water-tightness performance as explained as follows.
In consideration of erection costs and field quality control problems, the unitized panel system has gained overwhelming popularity in the industry in recent years. For a conventional unitized curtain wall system, a wall unit may span the height of one floor with split vertical mullions anchored at each floor slab. In this arrangement, the vertical grid line along the split mullion must be continuous from the bottom to the top and the horizontal grid line near the anchoring location must be continuous around the building.
For an Airloop curtain wall system, a unit may span between two adjacent continuous vertical whole mullions and is supported on the mullions. In this arrangement, the vertical grid line along a mullion must be continuous from the bottom to the top but the architect will have freedom in designing the horizontal grid line arrangement.
For a conventional unitized or Airloop window wall system, a unit may span between floor slabs with a split mullion and be supported by a horizontal base and ceiling tracks. In this arrangement, the horizontal grid lines along the base and ceiling tracks must be continuous around the building and the vertical grid lines can be offset from floor to floor.
The above grid line limitations are required due to considerations of wind load resistance. It is highly desirable to provide freedom for an architect in designing the grid line arrangement within a wall unit. Systems that do not allow for division within a wall unit are limited in their ability to allow for flexibility in designing the grid line pattern. For example, if the architect desires a grid line pattern with alternating narrow and wide panels as an exterior aesthetic feature, then the uneven mullion spacings would look odd from the interior. In addition, incorporation of the narrow units would result in major cost increases.
Therefore, grid line design freedom may be provided within a wall unit with horizontal and/or vertical dividers. However, the intersection points of the members including dividers and the perimeter frame members of the unit often become vulnerable to water leakage. For example, in most typical design conditions, the unit width is much smaller than the unit height; therefore, for wind load resistance, it is best for the horizontal divider to be continuous between the jamb frames of the unit and the vertical divider to be discontinuous at the horizontal divider. However, for water-tightness performance, it is best for the vertical divider to be continuous for easy downward water drainage. This presents a difficult decision to be made between the two options. In addition, once the choice has been made, there is no aesthetic freedom of choice for the intersection points.
In addition, the horizontal and/or vertical dividers create additional member intersection points which are vulnerable to water leakage due to the need for corner caulking and questionable long-term corner sealing integrity caused by joint stresses produced by thermal, wind, and seismic loads.